Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

The Flexor Hallucis Longus Tendon: Anatomy and Function

The flexor hallucis longus (FHL) is a powerful deep muscle of the calf whose tendon travels behind the ankle, through a fibro-osseous tunnel beneath the sustentaculum tali of the calcaneus, and across the plantar foot to insert at the base of the distal phalanx of the great toe. Its primary function is powerful plantarflexion (downward bending) of the big toe during push-off in walking and running—and critically, during the relevé (rising on the ball of the foot) position in dance. The FHL is sometimes called the “Achilles tendon of the foot” for dancers because of its central role in their technique and the severity of complications from its injury.

Causes and Mechanism of FHL Tendon Tears

FHL tendon pathology ranges from tenosynovitis (inflammation within the tendon sheath) to partial and complete tears. Tenosynovitis is the most common presentation, typically from repetitive plantarflexion overload in dancers, gymnasts, and running athletes. Partial tears occur from acute forced dorsiflexion of the big toe (such as landing from a jump with the toe extended), or from chronic degenerative changes in the tendon from years of overuse. Complete FHL tears are uncommon but devastating—they produce inability to actively plantarflex the interphalangeal joint of the great toe and significant weakness of push-off.

Symptoms and Diagnosis

FHL tenosynovitis produces posterior medial ankle pain that worsens with plantarflexion and is reproduced by resisted big toe plantarflexion against examiner resistance. A distinctive finding is a “trigger toe” phenomenon—the tendon catching in its fibro-osseous tunnel during ankle motion, creating a palpable snap or momentary locking of the toe. Partial tears cause pain and weakness with a positive Thompson-like squeeze test at the midportion of the tendon. Complete tears result in inability to actively flex the big toe IP joint.

MRI of the ankle and foot is the imaging modality of choice, demonstrating tenosynovial fluid, intratendinous signal changes indicating degeneration or partial tear, or complete tendon discontinuity. Ultrasound provides real-time dynamic assessment and can demonstrate the trigger phenomenon during active ankle motion.

Conservative Treatment for FHL Tenosynovitis

Tenosynovitis is initially managed with relative rest from provocative activities, NSAIDs, and a short course of immobilization in a walking boot. Physical therapy addresses FHL eccentric strengthening, posterior ankle stretching, and gait retraining. Ultrasound-guided corticosteroid injection into the FHL tendon sheath (not the tendon itself) reduces inflammation and allows return to training. Most cases of tenosynovitis resolve with 4–8 weeks of appropriate conservative management.

Surgical Treatment: FHL Tendon Repair and Release

Surgery is indicated for complete tears, large partial tears with persistent symptoms, and trigger toe that fails conservative management. For tenosynovitis and trigger toe, release of the fibro-osseous tunnel under the sustentaculum tali (performed open or endoscopically) eliminates the mechanical entrapment. Complete or large partial tears require primary repair—reapproximating the torn tendon ends with heavy non-absorbable suture using a Krackow or modified locking technique. When significant tissue is lost or the tendon is severely degenerated, reconstruction using flexor digitorum longus tendon transfer maintains big toe plantarflexion strength. Surgery is performed under ankle block anesthesia as an outpatient procedure.

Recovery after FHL repair involves 6 weeks non-weight-bearing in a short leg cast, followed by progressive weight-bearing in a boot. Dancers typically return to pointe work at 4–6 months; running athletes at 3–5 months depending on the extent of repair. Physical therapy addressing FHL strengthening, ankle range of motion, and sport-specific retraining is essential for full functional recovery.

Tendon Tear Surgery in Michigan

Flexor hallucis longus (FHL) tendon tears cause pain behind the ankle and big toe weakness, particularly in dancers and runners. Dr. Tom Biernacki provides expert diagnosis with MRI and ultrasound, along with both surgical and conservative treatment at Balance Foot & Ankle.

Learn About Our Surgical Options | Book Your Appointment | Call (810) 206-1402

Clinical References

1. Hamilton WG, et al. “Stenosing tenosynovitis of the flexor hallucis longus tendon and posterior impingement upon the os trigonum in ballet dancers.” Foot Ankle. 1993;14(3):149-155.
2. Michelson J, Dunn L. “Tenosynovitis of the flexor hallucis longus: a clinical study of the spectrum of presentation and treatment.” Foot Ankle Int. 2005;26(4):291-303.
3. Lo LD, et al. “MR imaging findings of entrapment of the flexor hallucis longus tendon.” AJR Am J Roentgenol. 2001;176(5):1145-1148.

Dr. Tom Biernacki, DPM

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Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.